Method and apparatus for the detection of hydrogen using a metal alloy

ABSTRACT

A hydrogen sensitive metal alloy contains palladium and titanium to provide a larger change in electrical resistance when exposed to the presence of hydrogen. The alloy is deposited on a substrate and a thin film and connected across electrical circuitry to provide a sensor device that can be used for improved sensitivity and accuracy of hydrogen detection.

This is a divisional of application(s) Ser. No. 08/366,645 filed on Dec.30, 1994, which is now U.S. Pat. No. 5,520,753.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the U.S.Government and may be manufactured and used by or for the U.S.Government without the payment of any royalties thereon or therefor.

FIELD OF THE INVENTION

This invention relates to a PdTi alloy in general and more particularlyto a hydrogen sensitive PdTi metal alloy for use in hydrogen sensitiveapplications, such as hydrogen sensitive resistors, metal oxidesemiconductor field effect transistors (MOSFET) and Schottky diodestructures.

BACKGROUND

Providing a material that can accurately detect the presence of hydrogenor hydrocarbons is desirable. Hydrogen sensitive metals undergo a change(e.g., a change in resistance) that can be detected when interactingwith hydrogen. For example, as hydrogen dissociates on a surface of ametal and migrates into the interior of the metal, the electricalresistance of the metal is changed. Usually, this increases theelectrical resistance of the metal. Similarly if the hydrogendissociates on the surface of a metal that is part of an electricalcircuit, then the electrical properties of the entire circuit areaffected.

Several metals and metal alloys have applications as hydrogen sensitivemetals. Palladium and alloys of palladium containing silver (forexample, PdAg) are known hydrogen sensitive metals. It is known to use apalladium resistor as a hydrogen sensor. The resistor is formed bydepositing palladium on a substrate. As hydrogen is absorbed by thepalladium, the resistance of the metal changes. The change inresistivity can then be detected (e.g., by an electrical circuitconnected to the palladium resistor). Additionally, it is known to usePd or PdAg as a gate in a MOSFET or Schottky diode device. The detectionof hydrogen by the gate triggers changes in the electronic properties ofthe device.

In both of these applications, the Pd and PdAg are sensitive to thepresence of hydrogen. The use of these materials, however, does havelimitations. For instance, in the palladium resistor as the palladiumdissociates and absorbs hydrogen, the palladium undergoes a phasetransformation. This causes hysteresis. Furthermore, this phasetransformation may damage the layer of palladium. Similarly, the PdAgfilm experiences a phase transformation. This occurs at higherconcentrations of hydrogen. The presence of the silver does, however,reduce damage to the film because the film is more resilient.

It is desirable for a hydrogen sensitive metal in the presence ofhydrogen to experience a large change in resistivity without .undergoinga phase transformation. Furthermore, it is desirable for the change tobe repeatable (i.e., the sensing metal can be used for multipleexposures to hydrogen).

This has been considered by Hughes et al., in "Wide Range H₂ SensorUsing Catalytic Alloys," presented at the 183rd Meeting of theElectrochemical Society, May 1993. Hughes et al. used an alloy ofpalladium and nickel, particularly Pd13%Ni, as a hydrogen sensitiveresistor. This material experienced nearly a 10% change in resistancewhen exposed to an environment consisting of 100% hydrogen.Additionally, the Pd13%Ni alloy did not undergo a phase transformationand is repeatable. The use of Pd13%Ni alloy as a hydrogen resistor,however, is limited because the alloy experiences a small change inresistance at low amounts of hydrogen (e.g., when exposed to anenvironment consisting of 10% hydrogen, the change in resistance issmall, approximately 1%).

Small changes in resistance may also be attributed to fluctuations intemperature. As a result, it is undesirable to use a material whichexperiences only a small change in resistance (e.g., less than 1%, asfor example, exhibited by the Pd13%Ni alloy) when exposed to low amountsof hydrogen because the change of resistance is similar to those changescaused by temperature fluctuations. It would be difficult to determinewhether the change in resistance is a result of the presence of hydrogenor a fluctuation in temperature. Unless strict temperature control ofthe resistor is possible, these materials are not acceptable fordetecting small amounts of hydrogen.

An alloy of palladium and chromium, particularly Pd13%Cr, has beentested as a hydrogen sensitive resistor. The Pd13%Cr alloy also did notundergo a phase transformation when .exposed to a hydrogen environment.Additionally, the detection of hydrogen is repeatable. The Pd13%Cralloy, however, experiences only a 1% change in resistance when exposedto an environment of 100% hydrogen. The use of this material in thisform would therefore be unacceptable at both low and high concentrationsof hydrogen for reasons discussed above.

Further, the number of alloys available for hydrogen detection islimited. Each alloy has its own sensitivity to hydrogen, hydrocarbons,and poisons. It is desirable to have a wide range of alloys available toenable detection of hydrogen bearing gases in a wide variety ofenvironments and temperatures.

Metal alloys containing palladium and titanium are known. For example,U.S. Pat. No. 4,082,900 to Shimogori et al. discloses a Ti--Pd alloycontaining 0.1 to 0.2% of palladium. The addition of palladium reducescrevice corrosion and embrittlement by absorbing hydrogen.

U.S. Pat. No. 4,139,373 to Norton discloses a Ti alloy containinganother metal such as palladium. The alloy consists of 60 to 94 weight %Ti and 6 to 40 weight % of at least one additional metal which includespalladium. The addition of palladium to the alloy reduces the corrosionrate and improves the electrical conductivity.

U.S. Pat. No. 4,536,196 to Harris discloses alloy coated with a layer oftitanium. A membrane of the alloy with the titanium coating is used fordiffusing hydrogen from a mixture of gases.

U.S. Pat. No. 4,666,666 to Taki et al. discloses a Ti alloy having smallamounts of Pd (i.e., between 0.005% to 2.0% by weight ). The alloy hasimproved corrosion resistance and improved resistance to hydrogenabsorption.

U.S. Pat. No. 4,719,081 to Mizuhara discloses a Pd alloy containing Tifor use in joining ceramic metals. The alloy includes 65 to 98 weight %palladium, 1 to 20% nickel, 0.5 to 20% chromium, 0.5 to 10 weight % Tior Zr and 0 to 10% molybdenum.

U.S. Pat. No. 4,728,580 to Grasselli et al. discloses an amorphous metalalloy that may contain Pd and Ti. The alloy is used for reversiblehydrogen gas storage.

None of the above mentioned U.S. patents, however, disclose the use of aPdTi metal alloy as a hydrogen sensor.

SUMMARY OF THE INVENTION

To solve the above and other problems, the present invention is directedto a hydrogen sensitive metal alloy containing palladium and titaniumthat has an increased change in electrical resistance in the presence ofhydrogen. The PdTi alloy will not undergo a phase transformation whenexposed to an environment of hydrogen. Furthermore, the hydrogensensitive PdTi alloy will experience a change in resistance uponexposure to hydrogen. This resistance change is present even afterrepeated exposure to environments containing, hydrogen The titanium inthe palladium acts as trapping sites for hydrogen. This reduces thediffusion of hydrogen through the alloy and yields a larger change inresistance in the presence of hydrogen. Further, Ti absorbs oxygen. Ashydrogen enters the metal, it reacts with the oxygen the Ti and removesthe oxygen from the alloy. This effect also changes the resistance ofthe alloy. Therefore, the sensitivity of the alloy comes from bothhydrogen being absorbed by the Pd but also oxygen being removed from theTi.

A thin film material of the hydrogen sensitive PdTi metal alloy inaccordance with embodiments of the present invention may be prepared bysputtering the palladium and titanium from multiple or single targets.Atomic particles of palladium and titanium are propelled onto asubstrate to form a thin film of the alloy. This technique produces analloy having a fine grain size. Other methods may also be employed toform the alloys (e.g., electron beam evaporation, thermal evaporation,etc.). After formation of the PdTi alloy, it may be annealed to improvehomogeneity of the alloy (i.e., to ensure that the palladium andtitanium are evenly distributed throughout the .alloy) as well as removeimpurities from the surface of the film. Furthermore, the alloy may alsobe produced in other forms (e.g., bulk materials such as wire). Thesecan be formed using numerous methods (e.g., extrusion and drawing).

A thin film of the PdTi alloy according to the present invention ispreferably sputtered then annealed. The alloy displays a change inresistance up to 18% when exposed to concentrations Of hydrogen. Thischange in resistance is a considerable improvement over the resistancechanges of the known hydrogen sensitive materials.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE schematically depicts a hydrogen detector having asubstrate with the PdTi alloy deposited thereon.

DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned above, metal alloys according to embodiments of the presentinvention incorporate palladium and titanium.

The base metal of the alloy is palladium. Palladium is capable ofabsorbing hydrogen. Additionally, is able to diffuse through palladiumreadily. As discussed above, palladium, alone, is not a suitablehydrogen sensor. Olsen et al. in "Palladium and Titanium Thin Films asProbes for Determination of Hydrogen in Helium," Anal. Chem., Vol. 49,No. 6, 1977, indicate that titanium is also capable of absorbinghydrogen. The hydrogen, however, does not migrate as readily through thetitanium. Yoshihara et al. in "The Diffusivity of Hydrogen inPalladium-Based Solid Solutions," Acta. Metall., Vol. 30, 1982 disclosethat the inclusion of titanium will decrease mobility of hydrogen inpalladium.

According to embodiments of the present invention, alloys of PdTi can beprepared which exhibit greater changes in electrical resistivity whenexposed to concentrations of hydrogen. With greater changes inresistivity at lower concentrations of hydrogen, the PdTi alloysaccording to embodiments of the present invention are more reliable indetecting the presence of hydrogen.

Since both metals are hydrogen sensitive, the relative concentration ofeach could range from above 0 to below 100% such as 1-99:99-1%. Withproper preconditioning, the alloy will still be hydrogen or hydrocarbonsensitive. The exact alloy ratio used will depend on the application. Ifthe sensor will be exposed to high concentrations of hydrogen, theamount of Ti in the alloy will be increased. If the sensor needs todetect hydrogen at low concentrations, the amount of Pd in the alloywill be increased. In preferred embodiments, the alloy contains between50 or 60 and 99 atomic % Pd. In a preferred form, the alloy containsbetween 70 and 98 atomic % Pd. In another preferred form, the alloycontains between 90 and 98 atomic % Pd.

According to one embodiment of the present invention, the PdTi alloy isformed by sputtering. Atomic particles of palladium and titanium areshot from separate targets (although a single target, can be used) ontoa substrate. The sputtering rates can be varied to vary the amount ofeach material present in the alloy. The Pd can be sputtered at a powerbetween 50 W and 450 W. In the preferred form, the Pd is sputtered at apower between 75 W and 300 W. In the more preferred form, the Pd issputtered a power between 100 W and 200 W.

Since the Ti is oxygen sensitive, the amount of Ti in the alloy willalso depend on the desire to track the oxygen concentrationsimultaneously. The Ti can be sputtered at a power between 25 W and 250W. In the preferred form, the Ti is sputtered at a rate between 50 W and150 W. In the preferred form, the amount of palladium in the alloy isgreater than the amount of titanium. For hydrogen sensing applicationsin environments which vary from inert hydrogen to air, a preferred powerratio of Pd/Ti is 150 W/50 W which corresponds to approximately 95.6atomic % Pd and 4.4 atomic % Ti.

Additional materials may be present in the alloy. These additivesinclude other materials which are oxygen or hydrogen sensitive. Theadditives may also provide sensitivity to other gases. These additivesinclude elements such as Cr, Ru, Ag, Au, Zr, Cu, Ir, Al or Hf. Inpreferred embodiments, the alloy may contain up to 20 atomic % of theseelements. Other additives including Pt and Ni may also be used. Alloyscontaining these additives may have less than or greater than 20 atomic% of Pt or Ni.

The sputtered particles adhere to the substrate 1 and form a thin filmlayer 3 on a surface 2 of the substrate. A thin layer is preferred tomaximize response and recovery time. For example, the thin layer mayhave a thickness up to 3000 angstroms. A layer that is on the order of5000 angstroms may result in cracking of the layer. A large number ofmaterials may be used as a substrate. If the user wishes to provide anoxygen reservoir for the Ti, then an oxide such as Al₂ O₃ or SiO₂ may beused. If the migration of hydrogen into layers beneath the alloy is tobe avoided, then a layer of Si₃ N₄ or Au may be employed. For hightemperature applications, the use of SiC as a semiconductor or asubstrate may be used. With SiC as a substrate, the sensor structure maybe heated to at least 600° C. At these temperatures, hydrocarbonsdisassociate and are detectable. The addition of Ti allows the alloy toact like a catalyst. This structure should be able to detecthydrocarbons in an oxygen concentration varying environment.

The thin film may be annealed to ensure that the palladium and titaniumare evenly distributed throughout the alloy. The annealing process alsoremoves impurities from the surface of the film which may affect theresistivity of the alloy. The sample may be annealed in air, inertenvironments, or in a vacuum for several hours or for several days attemperatures from 100° C. to at least 500° C.

In a hydrogen detector containing the PdTi alloy shown in the FIGURE,the thin film layer 3 is connected to electrical circuitry 4. Changes inthe electrical resistance of the thin film layer of PdTi alloy aredetected by the electrical circuitry 4 to indicate the presence ofhydrogen.

EXAMPLE 1

A thin film of the PdTi metal alloy is deposited on a substrate usingtwo gun sputtering. The palladium and titanium particles are shot fromseparate targets onto the substrate. The palladium and titanium aresputtered at power of 100 w and 50 w respectively. This produces analloy containing approximately 90.6 atomic % Pd and 9.4 atomic % Ti. Thefilm is then annealed at 250° C. overnight (i.e., approximately 12hours). The film in an environment of 100 % hydrogen experiences achange in resistance of near 16 to 18%. As the hydrogen concentration isdecreased, the resistance of the alloy returns to near the base lineresistance (i.e., the resistance of the alloy prior to exposure tohydrogen).

EXAMPLE 2

The same procedure as in Example 1 is repeated except that the palladiumand titanium are sputtered at power of 300 W and 50 W respectively,which yields approximately 98.9 atomic % Pd and 1.1 atomic % Ti.Furthermore, the film is not annealed. When exposed in an environment of100% hydrogen, this alloy experiences a change in resistance near 8%.

EXAMPLE 3

A thin film of the PdTi metal alloy is deposited on a substrate usingtwo gun sputtering with the palladium and titanium sputtered at power of150 W and 50 W respectively. This produces an alloy containingapproximately 95.6 atomic % Pd and 4.4 atomic % Ti. The response of thefilm is then measured in 100% hydrogen recovering in air and innitrogen. After an initial cycling in hydrogen, the resistance change ofthe alloy to 100% hydrogen is near 6% when measured in flowing nitrogen.The cycling in hydrogen changes the baseline by less than 0.8% over 3cycles.

The sample is then annealed in-situ at 250° C. in air for four days. Thesensor properties improve dramatically. The resistance changes at roomtemperature in 100% hydrogen by approximately 16% with a very stablerecovery to a baseline in air. The sample is then exposed to 100%hydrogen and allowed to recover in pure nitrogen. The baseline innitrogen is slightly higher than that in air (near 1%) but recovers tothe air resistance baseline value after exposure to air. The change inresistance of the alloy is repeatable when repeatedly exposed toenvironments of hydrogen.

EXAMPLE 4

A thin film of the PdTi metal alloy is deposited on a substrate usingtwo gun sputtering with the palladium and titanium sputtered at power of200 W and 200 W respectively- This produces an alloy containingapproximately 71 atomic % Pd and 29 atomic % Ti. The alloy has a 1%resistance change to exposure of up to 100% hydrogen as sputtered. Afterannealing for 4 hours at 295° C., the alloy changes resistance byapproximately 7% upon exposure to 100% hydrogen.

EXAMPLE 5

A thin film of the PdTi metal alloy is deposited on a substrate usingtwo gun sputtering with the palladium and titanium sputtered at power of400 W and 100 W respectively. This produces an alloy containingapproximately 96.3 atomic % Pd and 3.7 atomic % Ti. The alloy is heatedto 100° C. overnight in air, reduced to 35° C., and then exposed to 100%hydrogen. The sensor has a large resistance change (approximately 33%)but also has a large increase in the baseline resistance value(approximately 25%). This indicates a hydrogen induced phase change. Thealloy temperature is returned to 100° C. and then cycled in anenvironment of 100% hydrogen then air. The resistance change in 100%hydrogen is reduced to 5% at this temperature. The alloy exhibits aslight drift in baseline resistance but there is no indication of aphase change at the higher temperature.

EXAMPLE 6

A thin film of the PdTi metal alloy is deposited on a substrate usingtwo gun sputtering with the palladium and titanium sputtered at power of200 W and 50 W respectively. This produces an alloy containingapproximately 97.5 atomic % Pd and 2.5 atomic % Ti. After an initialexposure up to 100% hydrogen, the alloy is then exposed to increasingconcentrations of hydrogen. The alloy is exposed to environments having10%, 50% and 100% hydrogen. The alloy experiences a resistance change by5.5%, 11.76% and 16.7% respectively. The alloy is a reliable indicatorat low concentrations of hydrogen.

The invention has been described with reference to the embodiments andexamples thereof which are intended to be illustrative. Various changesand modifications may be made without departing from the spirit andscope of the invention as defined in the following claims.

The invention claimed is:
 1. A hydrogen detector comprising:a substrate;a thin film of PdTi metal alloy deposited on said substrate, whereinsaid PdTi alloy contains between 50 and 99 atomic % Pd and between 1 and50% Ti, and wherein the PdTi alloy experiences a change in electricalresistance when exposed to hydrogen; and electrical circuitry connectedto said PdTi thin film, wherein said change in resistance is detected bysaid electrical circuitry.
 2. The hydrogen detector according to claim1, wherein the PdTi alloy experiences a change in resistivity of atleast about 5% when exposed to an environment of 10% hydrogen.
 3. Thehydrogen detector according to claim 1, wherein the PdTi alloyexperiences a change in resistivity at least 10% when exposed to anenvironment of 100% hydrogen.
 4. The hydrogen detector according toclaim 1, wherein said thin film of PdTi forms a gate of a metal oxidesemiconductor field effect transistor and said electrical circuitry isconnected to said MOSFET.
 5. The hydrogen detector according to claim 1,wherein said thin film of PdTi forms a gate of a Schottky diodestructure and said electrical circuitry is connected to said Schottkydiode.
 6. A method of forming a hydrogen detector, comprising the stepsof: providing a substrate;depositing a thin film of PdTi alloy on thesubstrate wherein the PdTi alloy contains between 50 and 99 atomic % Pdand between 1 and 50% Ti; and connecting electrical circuitry to saidthin film of PdTi alloy, whereby change in the resistance in said thinfilm of PdTi alloy is detected by said electrical circuitry.
 7. Themethod according to claim 6, wherein the PdTi alloy experiences a changein resistivity of at least about 5% when exposed to an environment of10% hydrogen.
 8. The method according to claim 6, wherein the PdTi alloyexperiences a change in electrical resistance of at least 10% whenexposed to an environment of 100% hydrogen.
 9. The method according toclaim 6, wherein the thin film of PdTi alloy is deposited by sputtering.10. The method according to claim 9, wherein the PdTi alloy is depositedby separately sputtering Pd and Ti from separate targets.
 11. The methodaccording to claim 10, wherein the Ti is sputtered at a power of between25 W and 250 W.
 12. The method according to claim 11, wherein the Ti issputtered at a power of between 50 W and 150 W.
 13. The method accordingto claim 10, wherein the Pd is sputtered at a power of between 50 W and450 W.
 14. The method according to claim 13, wherein the Pd is sputteredat a power of between 75 W and 300 W.
 15. The method according to claim14, wherein the Pd is sputtered at a power of between 100 W and 200 W.16. A method for detecting hydrogen and hydrocarbons, comprising thesteps of:providing a substrate having a thin film of PdTi alloy thereon;connecting electrical circuitry to said thin film of PdTi alloy;detecting changes in electrical resistance in said PdTi alloy by meansof said electrical circuitry to indicate the presence of hydrogen andhydrocarbons.
 17. The method according to claim 16, wherein the PdTistructure and said electrical circuitry is connected to said Schottkydiode.
 18. The method according to claim 16, wherein the PdTi alloyexperiences a change in electrical resistance of at least 5% whenexposed to an environment of 10% hydrogen.